Toto Aquia flush button problem

Based on advice in this forum and my water company rebate, I installed two Tota Aquia toilets. They have proven to be very nice and flush well. They have saved significantly on the water bill, as my wife does home daycare. The glossy interior makes cleaning the washdown-style toilet easy.

Now for my problem: the flush button behavior seems to have changed over the last year. One toilet will only flush a tiny bit of water if the flush button (small or large) is not held at the bottom of the stroke for a second or two. The other toilet does this to a lesser extent, but neither toilet did this when new. Pushing the button to the bottom of it's travel was enough to flush properly.

The flush volume seems to now have become timing-dependent, requiring a skilled operator to properly flush the toilet everytime. What gives? Although it is still possible to get a decent flush out of the toilets, I am annoyed that you have to 'know how to flush it', especially with the use by children

What can I do to get these things to work properly like they did when new?